Control system, especially for a non-linear process varying in time

ABSTRACT

A control system, especially for a non-linear process varying in time, includes a first controller, in particular a linear PID controller, which is able to be switched over via a switching input between the &#34;tracking&#34; and &#34;control&#34; operating modes, and a second controller, which is advantageously a fuzzy controller. One output of the second controller leads to the tracking input of the first controller; at another output, a switching signal (S) is output which determines the operating mode of the first controller. The control device is applied in process automation.

BACKGROUND OF THE INVENTION

The present invention relates generally to control systems, and moreparticularly to a control system for a non-linear process varying intime, which includes two controllers, one of which is a linear PIDcontroller, by means of which a manipulated variable (y) is determinedfrom a comparison between a reference variable (w) and a controlledvariable (x).

When conventional controllers, e.g. linear PID controllers, are used tocontrol real processes, various problems arise. For non-linearprocesses, in particular, the parameters of a linear controller are onlyoptimal for one operating point of the process. When working withdiscontinuous processes, changing operating points occurs when startingand stopping the process. The consequence may be an inadequate controlresponse when a linear controller is used. In the same way, time-variantprocesses, whose behavior is variable because of external influences,such as fluctuating raw material quality, are problematic for linearcontrollers. In addition, in many processes there are interactionsbetween individual process variables. When working with a conventionalclosed-loop control, the process is usually controlled withsingle-variable controllers. With this procedure, a plurality ofsingle-loop control systems are employed, one for each of the individualprocess variables, while the interactions are disregarded. As a result,in certain process states, the closed-loop control may develop anunsatisfactory response, or instabilities may even occur. To resolvethese problems, so-called higher control processes, e.g. with adaptivecontrollers, state controllers or model-supported controllers, are used.In practice, however, these higher control processes are only usedinfrequently, because their application presupposes extensivetheoretical knowledge, and presupposes a process for which amathematical model can be developed. The resultant outlay is often veryhigh; to some degree, it is even impossible to describe a processmathematically. Therefore, in practice, a system operator must oftenintervene in the process, because, based on his experience, he will beable to recognize process states where the desired process behavior isnot guaranteed by the conventional automatic control. The disadvantageassociated with this is that because system operators have differentlevels of experience or even because their job performance can changefrom day to day, the result is a process control that is notreproducible.

The essay, "Fuzzy Control--werkzeugunterstutzteFunktionsbaustein-Realisierung fur Automatisierungsgerate undProzeβleitsysteme" "Fuzzy Control--Realizing Functional Modules forAutomation Systems and Process Control Systems through the Support ofSoftware Tools"! by Dr. Hans-Peter Preuβ, Edmund Linzenkirchner andSteffen Alender, published in "atp" 34 (1992) issue 8, pp. 451-460,discusses avoiding these problems by adding a second controller to theconventional controller. It mentions using the output of the secondcontroller for correcting manipulated variables, in order to support theconventional controller, as needed. In addition, the parameters Kp andTv of the conventional PID controller are adapted with a furthercontroller to the changing process dynamics. When these additionalcontrollers are switched on, sudden changes in manipulated variablesoccur disadvantageously, which can lead to surge loads on the finalcontrolling elements and, thus, to their rapid deterioration.

The present invention is directed to the problem of developing a controlsystem, especially for a non-linear process varying in time, by usingseveral controllers where surge loads on the final controlling elementscaused by sudden changes in manipulated variables are avoided.

SUMMARY OF THE INVENTION

The present invention solves this problem by providing a control systemof the type mentioned at the outset, in which: (1) the first controlleris able to be switched over via a switching input between a "tracking"and a "control" operating mode, in the "tracking" operating mode, themanipulated variable (y) is specified by a variable (N) applied to atracking input, and when switching over to the "control" operating mode,the manipulated variable (y) is adjusted to conform with the parametersof the first controller so that no sudden change in manipulatedvariables occurs; (2) one output of the second controller leads to thetracking input of the first controller; and (3) a device is provided forgenerating a signal (S) during certain process states, which is fed tothe switching input of the first controller and determines its operatingmode.

One advantageous embodiment of the control system according to thepresent invention provides that when working with a discontinuousprocess, the signal (S) is generated so that the first controller isoperated close to a quasi-steady-state operating point in the "control"operating mode and, when starting and shutting down the process, thefirst controller is operated in the "tracking" operating mode.

Another advantageous embodiment of the control system according to thepresent invention provides that the second controller is a fuzzycontroller.

Yet another advantageous embodiment of the control system according tothe present invention provides that when the second controller is afuzzy controller the signal (S) fed to the switching input is generatedby the fuzzy controller.

Yet another advantageous embodiment of the control system according tothe present invention provides that, when the second controller is afuzzy controller, besides the reference variable (w) and the controlvariable (x), still other process variables (p) are supplied to thefuzzy controller.

The present invention has the advantage that, with comparatively littleoutlay, a reproducible, fully automatic process control can be achieved,where manual intervention by a system operator is no longer needed. Byusing a fuzzy controller as an additional controller, there is no needto construct exact mathematical models for problematic non-linear ortime-variant processes, which often turn out to be very costly. Theinvention has an especially advantageous effect on discontinuousprocesses, since the advantages of conventional controllers can beutilized close to a quasi-steady-state operating point, and theadvantages of non-linear controllers can be utilized when starting andshutting down the process. In accordance with the present invention, asmooth changeover of the control system is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE shows a block diagram of a closed-loop control circuitcomprising a control system in accordance with the present invention.

DETAILED DESCRIPTION

A manipulated variable y for a process 2 is generated using aconventional controller 1. A suitable measuring transducer is used todetect a controlled variable x of the process 2 which is then subtractedin a comparator 3 from a reference variable w to produce a systemdeviation xD. This system deviation xD is supplied, in turn, to thecontroller 1 in order to close the control loop. The controller 1 may beoperated in two ways: in the "control" operating mode, in which themanipulated variable y is defined solely by the function of thecontroller 1 in dependence upon the system deviation xD, and in the"tracking" operating mode, in which the manipulated variable y ispredetermined by a variable N being applied to a tracking input. Aswitching signal S, which is applied to a switching input of thecontroller 1, is used to switch over the operating mode. The two signalsN and S are generated by a fuzzy controller 4 from the referencevariable w, the controlled variable x, and other process variables p.While the controller 1 is effective within a narrow range around asteady-state operating point, when working with the fuzzy controller 4,empirical strategies of the system operators are realized when startingand shutting down the process. By this means, a fully automatic,reproducible process control is achieved.

The designing of a fuzzy controller can be roughly divided into threesteps: defining of the input/output variables; defining of linguisticvalues for the input/output variables by establishing membershipfunctions; and defining rules while using the linguistic values forrealizing the desired strategy. Therefore, to be able to realize theempirical strategies of system operators in the fuzzy controller 4,first those process variables must be determined, which, when evaluated,make it possible to recognize the process states requiring manualintervention. These represent the input variables for the fuzzycontroller 4. If variables are relevant for assessing the process state,which are able to be formed through calculations (e.g. differentiation,combining several process variables mathematically, or filtering), thenthese calculations are realized as a conventional preprocessing, and theresults are supplied as input variables to the fuzzy controller. Thevariables for intervening in the process 2 are given by thosecontrollers on which the manual interventions take place. Thesevariables are the output variables of the fuzzy controller 4. Afterthat, one determines the value ranges for all input/output variables ofthe fuzzy controller 4, which make it possible to recognize the processstates in which manual interventions can follow or into which the rangeof manipulated variables can be subdivided in conformance with manualinterventions that took place previously. Linguistic terms, e.g."small", "medium" or "large" are assigned to these value ranges. Usingthe above-defined input/output variables and value ranges, the empiricalstrategies carried out up until this point are then described in theform of rules, which are comprised in each case of a conditionalcomponent, in which linguistic terms for value ranges of input variablesof the fuzzy controller 4 are combined with one another by an AND- or anOR-operation, and of an inference component, in which the desiredintervention in the process 2 is described, likewise using linguisticdescriptions of value ranges of the output variables. These value rangesare implemented as fuzzy quantities in the form of membership functionsfor fuzzification of the input variables or for defuzzification of theoutput variables. A special case is the switch-over signal S forswitching over the operating modes of the controller 1, whose valueranges correspond to sharp quantities. The determined empirical strategyis implemented in the form of fuzzy rules in a control action.

To guarantee a smooth transition from fuzzy control to conventionalcontrol, immediately after the changeover, the conventional controller 1takes on the variable N last specified by the fuzzy controller 4 at thetracking input as a manipulated variable y. When working with a PIDcontroller, this can be achieved in that the integral-action componentof the controller, corrected by the proportional and differentialcomponent, is adapted to the variable N in conformance with thecharacteristic curve of xD. The transfer from conventional control tofuzzy control is established in the fuzzy controller 4 by the membershipfunctions. A smooth transition can be effected here by optimizing themembership functions in the transfer range.

I claim:
 1. A control system comprising:a) a first controllerdetermining a manipulated variable (y) from a comparison between areference variable (w) and a controlled variable (x), said firstcontroller including a tracking operating mode and a control operatingmode, including a tracking input, and including a switching input forselecting either the tracking or the control operating mode, wherein inthe tracking operating mode the manipulated variable (y) is specified bya variable (N) applied to the tracking input, and when switching over tothe control operating mode, the manipulated variable (y) is adjusted toconform with parameters of the first controller so that no sudden changein the manipulated variable occurs; and b) a second controller having afirst output coupled to the tracking input of the first controller, andhaving a second output coupled to the switching input of the firstcontroller, said second controller generating a signal (S) duringcertain process states, which signal (S) is output via the second outputto the switching input of the first controller and determines theoperating mode of the first controller.
 2. The control system accordingto claim 1, wherein when working with a discontinuous process, thesignal (S) is generated so that the first controller is operated closeto a quasi-steady-state operating point in the control operating modeand, when starting and stopping the process, the first controller isoperated in the tracking operating mode.
 3. The control system accordingto claim 2, wherein the second controller comprises a fuzzy controller.4. The control system according to claim 3, wherein the signal (S) fedto the switching input is generated by the fuzzy controller.
 5. Thecontrol system according to claim 4, wherein in addition to thereference variable (w) and the control variable (x), other processvariables (p) are supplied to the fuzzy controller.
 6. A control systemfor a non-linear process in time comprising:a) a linear PID controllerdetermining a manipulated variable (y) from a comparison between areference variable (w) and a controlled variable (x), said PIDcontroller including a tracking operating mode and a control operatingmode, including a tracking input, and including a switching input forselecting either the tracking or the control operating mode, wherein inthe tracking operating mode the manipulated variable (y) is specified bya variable (N) applied to the tracking input, and when switching over tothe control operating mode, the manipulated variable (y) is adjusted toconform with parameters of the PID controller so that no sudden changein the manipulated variable occurs; and b) an additional controllerhaving a first output coupled to the tracking input of the PIDcontroller, and having a second output coupled to the switching input ofthe PID controller, said additional controller generating a signal (S)during certain process states, which signal (S) is output via the secondoutput to the switching input of the PID controller and determines theoperating mode of the PID controller.
 7. The control system according toclaim 6, wherein when working with a discontinuous process, the signal(S) is generated so that the PID controller is operated close to aquasi-steady-state operating point in the control operating mode and,when starting and stopping the process, the PID controller is operatedin the tracking operating mode.
 8. The control system according to claim6, wherein the additional controller comprises a fuzzy controller. 9.The control system according to claim 8, wherein the signal (S) fed tothe switching input is generated by the fuzzy controller.
 10. Thecontrol system according to claim 9, wherein in addition to thereference variable (w) and the control variable (x), other processvariables (p) are supplied to the fuzzy controller.